1. Field of the Invention
The present invention relates to an optical information recording and/or reproducing apparatus such as a magneto-optical disc apparatus. More particularly, the invention relates to an optical information recording and/or reproducing apparatus which can perform simultaneous recording/reproducing or simultaneous erasing/recording/reproducing while forming two optical spots on a recording medium.
2. Related Background Art
The magneto-optical disc apparatus is known as a data file device, utilizing the features of mass storage, and non-contact and rapid access. If such an apparatus employs an optical head irradiating a single optical beam, data recording normally requires rotation of a magneto-optical disc for each of erasing old information, recording new information, and checking (or verifying) the newly recorded information, i.e., three rotations in total. Alternatively, the erasing of old information and the recording of new information can be done at one time during one rotation and verifying of reproduction during a next rotation. Thus, the conventional magneto-optical disc apparatus requires a waiting time for rotations of the magneto-optical disc, which is a hindrance against an improvement in data transfer rate.
There are optical information recording and/or reproducing apparatus which can perform simultaneous erasing/recording/reproducing or simultaneous recording/reproducing using an optical head irradiating a plurality of optical beams, for example, as suggested in Japanese Laid-open Patent Application No. 58-220247 (corresponding to U.S. Pat. No. 4,517,667) or in Japanese Laid-open Patent Application No. 64-82348. FIG. 1 is a schematic diagram to show an example of such a conventional magneto-optical recording and/or reproducing apparatus using an optical head for irradiating a plurality of beams.
In FIG. 1, a monolithic semiconductor laser array 1 has a first radiative point 1a and a second radiative point 1b on the same substrate, and beams of linearly polarized light emitted from the radiative points pass through nearly the same optical path to be focused on a magneto-optical disc 5. In more detail, the optical beams emitted from the laser array 1 are collimated by a collimating lens 2 and the collimated beams pass through a beam splitter 3 with a beam shaping portion and are then focused by an objective lens 4 to form two optical spots 13a, 13b on an information recording surface 6 in the magneto-optical disc 5. The radiative point 1a corresponds to the optical spot 13a and the radiative point 1b to the optical spot 13b. The two optical spots 13a, 13b are arranged such that the optical spot 13a advances and the optical spot 13b follows on the same information track in the information recording surface 6 in the magneto-optical disc 5 rotating in the direction of arrow 17. Beams of light reflected by the information recording surface 6 are again condensed by the objective lens 4 and then deflected by the beam splitter 3 with the beam shaping portion to be guided into a signal detecting optical system.
The signal detecting optical system is composed of a half wave plate 7, a condenser lens 8, a polarization beam splitter 9, and photo sensors 11, 12 each having a split light receiving region. The half wave plate 7 and the polarization beam splitter 9 function to equalize a quantity of light reaching the photo sensor 11 nearly with that reaching the photo sensor 12. The photo sensor 11 has receiving zones a1, b1, c1, d1, e1 while the photo sensor 12 has receiving zones a2, b2, c2, d2, e2. Optical spots 14a, 14b are formed on the photo sensor 11, corresponding to the optical spots 13a, 13b on the information recording surface 6 (i.e., corresponding to the radiative points 1a, 1b). Also, optical spots 15a, 15b are formed on the photo sensor 12, corresponding to the optical spots 13a, 13b on the information recording surface 6 (i.e., corresponding to the radiative points 1a, 1b).
In data recording, the radiative point 1a emits a beam to form the optical spot 13a with a recording power, and the radiative point 1b emits a beam to form the optical spot 13b with a reproduction power lower than the recording power. Further, an external magnetic field head 16 applies a magnetic field modulated by data information to a recording area. Then, obtained from the optical spot 13b for reproduction are servo signal information for so-called focusing and tracking, and an information reproduction signal for verifying information recorded by the optical spot 13a. This arrangement permits simultaneous achievement of erasing of old information, recording of new information, and reproduction for verifying the newly recorded information.
For normal data reproduction, only the radiative point 1b is activated to emit a beam to form the optical spot 13b with reproduction power.
Describing outputs from the receiving zones in the photo sensors by the same reference symbols as those for the receiving zones, various signals can be obtained by the following calculations: ##EQU1## Outputs from the receiving zones e1, e2 where the optical spots 14a, 15a are formed are normally unused.
FIG. 2 is a schematic diagram to show a modification of the signal detecting optical system in the conventional MO recording and/or reproducing apparatus using the optical head emitting a plurality of beams, as described above. This modification is different in the arrangement of receiving zones in the photo sensors 11, 12 from the arrangement in FIG. 1.
This modification is so arranged that in data recording, the radiative point 1a emits a beam to form the optical spot 13a with a recording power and the radiative point 1b emits a beam to form the optical spot 13b with a reproduction power lower than the recording power, and that an external magnetic field head (not shown) applies a magnetic field modulated by data information to a recording area. In this arrangement, servo signal information for so-called focusing and tracking is obtained from the optical spot 13a, while an information reproduction signal for verification of information recorded by the optical spot 13a is obtained from the optical spot 13b. This permits simultaneous achievement of erasing of old information, recording of new information, and reproduction for verifying the newly recorded information.
For normal data reproduction, only the radiative point 1a is activated to emit a beam to form the optical spot 13a with reproduction power.
Describing outputs from the receiving zones in the photo sensors by the same reference symbols as those for the receiving zones, various signals can be obtained by the following calculations: ##EQU2##
Incidentally, the optical system as shown in FIG. 1 is generally arranged to assure a recording power of 6 to 10 mW with such characteristics of a beam split film in the beam splitter 3 with a beam shaping portion that the transmittance is in the range of about 60 to 80% and the reflectivity in the range of about 20 to 40%. Accordingly, the reproduction power is about 1.2 mW, and, assuming the reflectivity of the magneto-optical disc is about 15% and attenuation by interposed optical elements is about 20%, a quantity of light reaching each photo sensor 11, 12 is as weak as about 0.02 mW. Also, an MO signal is further weaker, which is about 5% of the quantity of light reaching each sensor. A signal from each receiving zone in the photo sensor 11, 12 is amplified by a pre-amplifier (preamp) and transmitted to a signal processing system, as shown in the block diagram of FIG. 3. Therefore, the noise characteristics of the preamps as shown in FIG. 3 greatly affect the performance of reproducing the MO signal.
Nevertheless, the above conventional example as shown in FIG. 2 obtained the MO signals as follows: ##EQU3## Taking the photo sensor 11 as an example, a preamp is used for verify-reproduction-while-recording whereas four preamps for normal reproduction. Then, letting .alpha. [Ap-p] be a noise level for each preamp, noise caused by the preamps is .alpha. [Ap-p] in verify-reproduction-while-recording but 2.alpha. [Ap-p] by mean square in normal reproduction. Thus, the normal reproduction has a worse S/N ratio (Signal to Noise Ratio) than the verify-reproduction-while-recording has, which makes the verification less meaningful. Also, the noise caused by the preamps is proportional to a square root of the frequency band of the signal. On the other hand, the frequency band of the signal must be widened to improve the data transfer rate, which increases the noise caused by the preamps. An absolute value of S/N will be insufficient in a case of a plurality of preamps involved.
An increase in noise caused by the preamps can be prevented by a separate arrangement of a servo signal detecting optical system and a data information signal detecting optical system, though not shown. In this case, a quantity of light reaching the data information signal detecting system sensor is decreased by a quantity separated into the servo signal detecting system, which lowers S/N, increases the size of the apparatus, increases the cost, and makes assembling and adjustment complex.